Systems and methods for providing modulation of switchmode RF power amplifiers

ABSTRACT

Systems and methods are provided for generating an amplitude modulation signal to a switchmode power amplifier. A DC to DC switch is configured to receive a DC input voltage and to provide a DC output voltage. A low dropout regulator is configured to provide the amplitude modulation signal according to a modulation control signal received by the low dropout regulator. A control circuit is configured to establish a nominal operating power level for the power amplifier via the amplitude modulation signal and to maintain a minimum voltage difference between the DC output voltage and the low dropout regulator output. A modulator control circuit is configured to provide the modulation control signal to the low dropout regulator. The modulator control circuit provides the transition from a high amplitude to a low amplitude and a transition from the low amplitude to the high amplitude at configurable first and second slopes, respectively.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/961,089, filed Dec. 6, 2010, and entitled “Systems and Methods forProviding Modulation of Switchmode RF Power Amplifiers” the entirety ofwhich is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates generally to signal processing and moreparticularly to switchmode RF power amplifiers.

BACKGROUND

The Class E amplifier is a highly efficient switchmode power amplifier.In a Class E amplifier, the transistor is connected via a serial LCcircuit to the load and connected via a large inductor to the supplyvoltage. The supply voltage is connected to ground via a large capacitorto prevent any RF signals leaking into the supply. The Class E amplifieradds a capacitor between the transistor and ground and uses a definedinductor to connect to the supply voltage. The systems and methodsdescribed herein are operable with a class E amplifier as well as otheramplifier classes.

SUMMARY

In accordance with the teachings provided herein, systems and methodsare described for providing an amplitude modulation signal to aswitchmode amplifier. A DC to DC switch may be configured to receive aDC input voltage at one level and to provide a DC output voltage at adifferent level at high efficiency. A low dropout regulator (LDO) may beconfigured to provide the amplitude modulation signal according to amodulation control signal received by the low dropout regulator, the lowdropout regulator being powered by the DC output voltage. A controlcircuit may be connected to the DC to DC switch and the low dropoutregulator, where the control circuit is configured to establish anominal operating power level for the power amplifier via the amplitudemodulation signal and to maintain a minimum voltage difference betweenthe DC output voltage and LDO output. A modulator control circuit may beconfigured to provide the modulation control signal to the low dropoutregulator, the modulation control signal including transitions from ahigh amplitude to a low amplitude back to a high amplitude, where themodulator control circuit provides the transition from the highamplitude to the low amplitude at a configurable first slope, and themodulator control circuit provides the transition from the low amplitudeto the high amplitude at a configurable second slope.

As another example, a method of generating an amplitude modulationsignal to a switchmode power amplifier using a DC to DC switch, a lowdropout regulator, a control circuit, and a modulator control circuitmay include providing a DC input voltage to the DC to DC switch toprovide a DC output voltage. A minimum voltage difference may bemaintained between the DC output voltage and the low dropout regulatorusing a control circuit connected to the DC to DC switch and to the lowdropout regulator. A modulation signal may be provided to the lowdropout regulator using a modulator control circuit, where themodulation control signal includes transitions from a high amplitude toa low amplitude back to a high amplitude. The modulator control circuitmay provide the transition from the high amplitude to the low amplitudeat a configurable first slope, and the modulator control circuit mayprovide the transition from the low amplitude to the high amplitude at aconfigurable second slope.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,aspects, and advantages of the invention will become apparent from thedescription, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram depicting an example amplifier.

FIG. 2 depicts an example of a modulated AC output signal produced by anamplifier.

FIG. 3 is a block diagram depicting an example amplifier connected to atransmitter antenna.

FIG. 4 is a block diagram depicting an amplifier/antenna combinationdeployed in a medical environment.

FIGS. 5 and 6 depict an example signal mask for a bit sought by an RFIDcircuit in accordance with an RFID standard.

FIG. 7 is a graph depicting the spectrum of the square wave signaldepicted in FIG. 6.

FIGS. 8A and 8B depict graphs showing signals that traverse a maskutilizing slope control.

FIG. 9 depicts the spectrum of the signal of FIG. 8A.

FIG. 10 depicts an example system for controlling the amplitudemodulation signal provided to an amplifier.

FIG. 11 is a block diagram depicting example components of an amplitudemodulation signal generator.

FIGS. 12A and 12B illustrate a schematic depicting an exampleimplementation of an amplitude modulation signal generator.

FIG. 13 is a schematic depicting an example implementation of a controlcircuit.

FIGS. 14A and 14B illustrate a schematic depicting an exampleimplementation of a DC to DC switch.

FIGS. 15A and 15B illustrate a schematic depicting an exampleimplementation of a low dropout regulator.

FIGS. 16A and 16B illustrate a schematic depicting an example modulatorcontrol circuit.

DETAILED DESCRIPTION

FIG. 1 is a block diagram depicting an example amplifier. The amplifier102 is connected to a power node 110 and a ground node 106. The switchnode 104 provides a reference switch signal at a carrier frequency tooperate the switchmode power amplifier. The power node 1 may provide aDC voltage at a certain level. The amplifier 102 provides an AC outputsignal 108 based on a received amplitude modulation signal 110. The ACoutput signal 108 has a magnitude that is proportional to the amplitudemodulation signal 110 at a particular carrier frequency received at 104.An example amplifier may be a class E amplifier. It should be noted thatthe systems and methods described herein may also be utilized with othertypes and classes of amplifiers.

FIG. 2 depicts an example modulated AC output signal produced by anamplifier. The example AC output signal is propagated at a particular(carrier) frequency, where the carrier frequency is established by asignal at node 104 and amplitude set by node 110. The amplitude of theAC output signal is varied by the amplifier according to a receivedamplitude modulation signal. For example, when the amplitude modulationsignal is at a high level, then the AC output signal may be transmittedat a high magnitude (e.g., before time period ti and after time periodt₃). When the amplitude modulation is at a low level, then the AC outputsignal may be transmitted at a low magnitude (e.g., during time periodt₂). The AC output signal may also be transmitted at an intermediatemagnitude as dictated by the amplitude modulation signal (e.g., when theAC output signal is configured to differentiate between more than twolevels, during a transition between a high magnitude and a lowmagnitude). The amplifier may be a high speed device, wherein changes inthe amplitude modulation signal are quickly reflected in the AC outputsignal.

The example AC output signal has a variety of features. The AC outputsignal has a high magnitude at level a 202 and a low magnitude at alevel b 204. The difference between level a 202 and level b 204 is themodulation depth of the AC output signal. The transition between thehigh magnitude 202 and the low magnitude occurs at a first slope 206,and the transition between the low magnitude 204 and the high magnitude202 occurs at a second slope 208. The time period, t₂ 210, for a singlebit-length at the low magnitude is a dwell time.

An amplifier, such as the amplifier depicted in FIG. 1 may be utilizedin a variety of contexts. FIG. 3 is a block diagram depicting an exampleamplifier connected to a transmitter antenna. An amplifier 302 isconnected to a switch node 304 and a ground node 306. The amplifier 302outputs an AC output signal 308 that is proportional in magnitude to anamplitude modulation signal 310. The AC output signal 308 is provided toan antenna 312 that propagates an electromagnetic or other signal thatis proportional to the AC output signal 308.

Such a configuration may be used in a variety of implementations. FIG. 4is a block diagram depicting an amplifier/antenna combination deployedin a medical environment. The amplifier/antenna combination 402 is usedfor activating RFID circuits 404 located within the medical environment406 RFID circuits 404 may be programmed to listen for certain bitsequences, which may be represented by certain amplitude patterns of asignal at a particular frequency transmitted by the amplifier/antennacombination 402. When an RFID circuit 404 detects an appropriate signal,the RFID circuit 404 may respond, such as with a reply signal containingdata. The data may be representative of a variety of information, suchas a unique identifier of the RFID circuit that can be used to identifyan item to which the RFID circuit is attached. Other electronicinstruments may also be present in a medical environment 406, such asmedical devices 408 and pacemakers 410. Medical devices 408 andpacemakers 4 may be susceptible to interference produced by signalstransmitted by the amplifier/antenna combination 402. For example, aheart monitor medical device could erroneously recognize a signaltransmitted by the amplifier/antenna combination 402 as part of a heartbeat signal. Similarly, a pacemaker 410 may recognize a signal from theamplifier/antenna combination 402 as an irregular heartbeat and may takecorrective action, such as providing an electric shock to a heart towhich the pacemaker 410 is connected. Such a situation can be verydangerous.

It may be desirable to use RFID circuits 404 in a medical environment406. During a surgical procedure, items are often counted repeatedly bythe surgical staff in order to ensure that nothing is inadvertently leftinside of a patient's open body cavity. Items most commonly counted toprevent accidental retention include surgical sponges, towels, and smallinstruments. Retained items are known to cause dangerous complicationssuch as infection, pain, and even death.

Presently, there is a manual method widely employed to prevent retaineditems. This method consists of counting each item before, during, andafter surgery to ensure that starting counts reconcile with finalcounts. This method indirectly validates that nothing is retained insidethe body by accounting for all items used by the end of the surgery.Guidelines vary slightly, but in general these counts are standardizedacross hospitals in the United States, and they include a startingcount, post-operative count, and several intermediate counts at variousstages of the surgery. These counts must be maintained as an ongoingtally in order to account for new items introduced mid-procedure, suchas a new pack of sponges used to address unexpected bleeding.

Manual counts are often flawed and error-prone despite proactivemeasures such as standardization of counts and reactive punitivemeasures such as mandatory public reporting of retained object incidentsand imposition of fines on hospitals where such errors occur. Manualcounting is made difficult by the large number of items to be countedand the stressful chaotic operating room environment. Further riskfactors for retained objects include large patient body mass and thecolor, consistency, and adherence of sponges when used to absorb bloodand body fluids. Sponges are difficult to discern when soaked withblood, particularly in larger patients.

More recently, technological advances have been aimed at improving theprocess of manual counting and augmenting or replacing it with anautomated, machine-validated count.

One such system is a RFID based system which consists of a plurality ofmarker tags, each containing a unique data string, permanently attachedto each surgical item. In addition to a unique number, the marker tagsmay contain some data about the item to which they are attached, such asthe type of item, manufacturer information, etc. The system alsoconsists of a reader, which is configured to scan the tagged items,detecting their markers, and displaying information to the user aboutthe type and quantity of items detected.

RFID systems for surgical sponge and instrument management may beconfigured to both count and detect tagged items. Because RFID tagscontain unique data strings, they can be individually counted. Becausethe tags may communicate from a predetermined distance with noline-of-sight requirement and can effectively read through the humanbody, the patient may be scanned as a “safety check” for retained items.RFID systems may have multiple modules in communication with each otherthat are each configured for one of these various functions. Forexample, one module may be configured for validating the count ofsponges being entered into surgery, while another module may beconfigured for scanning the patient for retained items.

An element of these surgical RFID systems is the RFID reader circuitry.This circuitry is typically in communication with a processor that, inaddition to controlling the RFID reader, may also control a display,user controls, and other peripheral devices that contribute to theoverall functionality of the system. The RFID reader is responsible forpowering one or more antennas, thus creating a radiofrequency fieldwhich establishes communication with one or more marker tags. The readerissues a series of commands to these tags and receives and decodes theirresponses. These responses are passed to the processor, where they arefurther deciphered into information for display to the user, such as thequantity and type of items within the detection range of the antennas.

The RFID reader may be characterized by its carrier frequency, poweroutput, and modulation scheme. Each of these parameters may beconsidered from the standpoint of system performance as well asenvironmental, interoperability, and regulatory perspectives. Certaincarrier frequencies may be better-suited for particular environments.For example, to read through a human body to detect and identify RFIDtags, a carrier frequency may be selected that has low attenuationthrough fluids.

Power output is related to performance characteristics such as maximumread range of tags. Modulation, the system of variation in the carrierfrequency's amplitude or frequency to convey data, may determine howeffectively the tag can deliver its information to the reader. Eachparameter may be selected to optimize performance. However, in thesurgical environment there are multiple competing concerns that may workto prevent system designers from optimizing performance.

One issue that is faced in the surgical environment is that ofmaximizing read range of RFID tags while satisfying regulatoryrequirements of interoperability of electronic devices. Each devicedeployed in the operating room, unless exempt from regulations, issubject to a spectral analysis in some frequency range that includes itsoperating, or carrier, frequency. A series of power limits is applied,which vary by frequency. Viewed in the frequency domain, this set ofpower limits is called a spectral mask. A common characteristic of thesespectral masks is that they allow a relatively high power output at theoperating frequency of a device, but this limit drops off quickly aboveand below the operating frequency. When a device, such as an RFIDreader, operates by communicating bits of data, these bits are usuallycomprised of rapid changes in amplitude or frequency that represent onesand zeros. Using amplitude modulation as an example, ones and zeros arerepresented by changes in the amplitude of the carrier, where themagnitude of these changes is defined as the modulation depth.

Existing modulation techniques create square pulse shapes, which resultin harmonics that may have deleterious effects from a regulatory andinteroperability standpoint. For example, square pulses createunnecessary low frequency spectral components, visible as wider spreadaround the carrier frequency in the frequency domain, which usuallyrequire the designer of such a system to lower the overall transmittedpower in order to comply with the regulatory spectral mask.

Additionally, it has been shown that certain low frequency spectralcomponents, particularly those in the same ranges as biologicalfrequencies, may impact the operation of certain medical devices thatare tuned to detect such frequencies. For example, cardiac rhythmmanagement devices (CRMDs) are highly sensitive devices tuned to detectlow-level electrical impulses from the heart in the range of 0-100 kHz.In a surgical environment, devices with carrier frequencies in thisrange, those with modulation rates in this range, or those with otherlow-frequency spectral components in this range, are at risk ofinterfering with such devices.

FIG. 5 depicts an example signal mask 502 for a bit sought by an RFIDcircuit. An RFID circuit may wait in a dormant state until a signalhaving an expected amplitude pattern of one or more bits at a particularfrequency is detected. The mask of FIG. 5 shows the range over which asingle amplitude shift keying transition can occur. When a specificsignal pattern is detected, the RFID circuit is programmed to transmitresponse data. For example, the RFID circuit may transmit a uniqueidentifier of the RFID circuit as well as other data programmed onto theRFID circuit. The RFID circuit may utilize power from the receivedpolling signal that awakened the RFID circuit, or the RFID circuit mayinclude a battery or other power source to use in transmitting aresponse signal.

FIG. 6 depicts an example square wave signal within the example mask,where a pattern of bits falling within bit masks would awaken an RFIDcircuit. The example signal is received at the particular frequency at ahigh magnitude 602, transitions to a low magnitude 604, and transitionsback to a high magnitude 606, all within the mask 608 listened for bythe RFID circuit. Upon detecting such an expected bit pattern, an RFIDcircuit may be awakened to transmit data via an antenna responsive tothe RFID circuit.

By quickly changing the amplitude of the AC output signal from highmagnitude to low magnitude back to high magnitude, the spectral densityof the AC output signal may be quite large, outputting significantamounts of power at several frequencies outside of the particularcarrier frequency of the amplifier. FIG. 7 is a graph depicting thespectrum of the square wave signal depicted in FIG. 6. FIG. 7 depictsthe power of the square wave signal as a function of frequency.

The square wave signal includes a large power component at the carrierfrequency, shown in the center of the graph. However, the square wavesignal also includes significant power at other frequencies. The poweremitted at other frequencies can be problematic for a number of reasons.For example, the power at frequencies outside of the carrier frequenciesmay cause interference with other electronic devices, such as themedical devices and pacemakers depicted in FIG. 4. As another example,certain regulations may require limitation of power transmitted atcertain frequencies. For example, the Federal Communications Commission(FCC) may require that certain devices limit their emissions outside ofa permitted carrier frequency. Such a requirement may be implemented byrequiring devices to limit their emissions to fit within a mask. FIG. 7depicts an example regulatory mask 702 within which a device may berequired to remain. At several points 704, the square wave signal ofFIG. 6 transmits power outside of (above) the mask 702.

FIGS. 8A and 8B depict graphs showing signals that traverse a maskutilizing slope control. As shown in FIG. 8A, there are alternate waysfor a signal at the particular carrier frequency to remain within a mask802, such as one sought by an RFID circuit. FIG. 8A depicts a signal atthe carrier frequency that utilizes a moderate slope 804 to transitionfrom a high magnitude to a low magnitude, a dwell time shown at 806, anda gradual slope 808 from the low magnitude to the high magnitude. Thesignal depicted in FIG. 8A remains within the mask 802, making itdetectable by devices listening for such a signal. However, the signaldepicted in FIG. 8A will have a different spectrum than the square wavesignal depicted in FIG. 6.

FIG. 8B depicts an alternate signal at the particular frequency thatremains within the mask 810. The signal in FIG. 8B includes a steepslope 812 in transitioning from the high magnitude to the low magnitude,a dwell time shown at 814, and a fairly steep slope 816 in transitioningfrom the low magnitude to the high magnitude. The spectrum of the signalin FIG. 8B differs from that of the signal depicted in FIG. 8A as wellas that of the square wave signal depicted in FIG. 6.

FIG. 9 depicts the spectrum of the signal of FIG. 8 A. As discussedabove, the addition of slope control, as opposed to a fast transitionfrom high magnitude to low magnitude back to high magnitude, enablesmodification of the spectrum of the signal. This is illustrated by theapplication of a similar mask 902 in FIG. 9 as the mask applied to thesquare wave signal spectrum in FIG. 7. While the square wave signalspectrum in FIG. 7 violated the mask at several points, the spectrum ofthe slope controlled signal remains within the example mask 902.

FIG. 10 depicts an example system for controlling the amplitudemodulation signal provided to an amplifier. Previously, systems may havedirectly applied a data signal to an amplifier as the amplitudemodulation signal. For example, directly applying a data signaltransitioning from a 1 to a 0 back to a 1 could create the square wavesignal at the particular carrier frequency shown in FIG. 6. Such asystem may be suboptimal based on power emitted outside of theparticular carrier frequency.

FIG. 10 depicts a system that enables configurable slope control thatfacilitates shaping of the spectrum of the signal. A data signal may bereceived by an amplitude modulation signal generator 1002. The receiveddata signal may be modified prior to transmitting the data signal to theamplifier 1004 for transmission as an AC output signal 1006. Forexample, the slope of transitions from a first magnitude to a secondmagnitude and from the second magnitude to the first or anothermagnitude may be adjusted. Other parameters of the data signal, such asa dwell time for a bit may also be adjusted by the amplitude modulationsignal generator 1002.

FIG. 11 is a block diagram depicting example components of an amplitudemodulation signal generator. The amplitude modulation signal generator1102 generates an amplitude modulation signal 1104, such as to aswitchmode power amplifier. The amplitude modulation signal generatorincludes a DC to DC switch 1106 configured to receive a DC input voltageand to provide a DC output voltage 1108. A low dropout regulator 1110 isconfigured to provide the amplitude modulation signal 1104 according toa modulation control signal 1112 received by the low dropout regulator1110. The low dropout regulator is powered by the DC output voltage1108. A control circuit 1114 is connected to the DC to DC switch 1106and the low dropout regulator 1110. The control circuit 1114 isconfigured to establish a nominal operating power level for the poweramplifier via the amplitude modulation signal 1104 and to maintain aminimum voltage difference between the DC output voltage 1108 and thelow dropout regulator 1110. A modulator control circuit 1116 isconfigured to provide the modulation control signal 1112 to the lowdropout regulator 1110. The modulation control signal 1112 includestransitions from a high amplitude to a low amplitude back to a highamplitude, such as, for example, dictated by a data input signal 1118.The modulator control circuit 1116 provides the transition from the highamplitude to the low amplitude at a configurable first slope, and themodulator control circuit 1116 provides the transition from the lowamplitude to the high amplitude at a configurable second slope. Themodulator control circuit 1116 may also control other aspects of themodulation control signal 1112 such as dwell times for a single bit anda modulation depth between different signaling levels.

FIGS. 12A and 12B illustrate a schematic depicting an exampleimplementation of an amplitude modulation signal generator. A DC to DCswitch 1202 receives an input voltage and provides a DC output voltagewith very efficient conversion. The DC to DC switch 1202 provides the DCoutput voltage to a low dropout regulator 1206 for power. The lowdropout regulator 1206 may have a high switching speed capabilitycompared to the DC to DC switch 1202. Controlling the voltage differencebetween the output 1202 and the output of the low dropout regulator 12to maintain a minimum operating voltage difference (e.g., 0.5 volts)enables the system to maintain a high efficiency.

A modulator control circuit 1208 provides the modulation control signalto the low dropout regulator 1206. The modulator control circuit 1208provides shaping of a data signal to be output from the amplitudemodulation signal generator such as slopes for transitions fromsignaling levels, modulation depth between signaling levels, dwell timesfor bits, as well as others. The low dropout regulator 1206 provides theAC output signal, such as to an amplifier, at the node labeled +PA_VDD1210.

FIG. 13 is a schematic depicting an example implementation of a controlcircuit. The control circuit 1302 may include a digital to analogconverter 1304 that is digitally controlled by a master device (notshown). The control circuit 1302 may establish a nominal output power ofan amplifier (not shown), which is controlled by the amplitudemodulation signal at +PA_VDD shown in FIGS. 12 and 1. The controlcircuit 1302 controls the output of the DC to DC switch.

FIGS. 14A and 14B illustrate a schematic depicting an exampleimplementation of a DC to DC switch. The control circuit controls theoutput of the DC to DC switch 1402 by introducing a current into afeedback of the DC to DC switch via a voltage output divide by resistorR78 at 1404. With reference back to FIG. 13, the digital to analogconverter 1304 of the control circuit 1302 also supplies a voltage tooperational amplifier U84 at 1306, which introduces a current into afeedback node of a high speed low dropout regulator.

FIGS. 15A and 15B illustrate a schematic depicting an exampleimplementation of a low dropout regulator. The low dropout regulator 102receives the signal from the control circuit at resistor R16, shown at104. The input from the control circuit received at 104 may beconfigured to maintain a minimum voltage difference between the outputof the DC to DC switch, shown in FIGS. 14A and 14B and the input to thelow dropout regulator 102, such as at 0.V. This configuration enableshigh overall efficiency because the voltage drop of the low dropoutregulator is maintained at 0.V below the DC to DC switch output.

FIGS. 16A and 16B illustrate a schematic depicting an example modulatorcontrol circuit. The modulator control circuit 1600 provides modulationand wave shaping of the amplifier output. The modulation and waveshaping may be implemented using a high speed digital to analogconverter U32, shown at 1602. U32 1602 introduces a second current intothe feedback node of the low dropout regulator at 1506. The low dropoutregulator reacts by raising or lowering the voltage at the output node+PA_VDD at 108. A master device (not shown) can provide data to betransmitted to the output node +PA_VDD 108 via the modulator controlcircuit. The master device can also dictate certain parameters of thewaveform representing that data to the modulation control circuit. Forexample, the master device may dictate slopes for changes in amplitudesignaling levels, modulation depths, dwell times, and other parametersof the signal. These parameters may be input to the modulation controlcircuit in a number of ways. For example, the parameters may be dictateddigitally or via an analog control, such as via a switch or knobsetting.

Many variations of an amplitude modulation signal generator arecontemplated by this disclosure. For example, a DC to DC switch mayutilize a variety of parts including an LT3693, an LM5576, an LM22678,and an LM 20343. A control circuit may utilize a variety of partsincluding an LTC2630, an ADS5443, a DAC121S101, and a TLV5638. A lowdropout regulator may utilize a variety of parts including an LT1575. Amodulator control circuit may utilize a variety of parts including anAD5424, an AD9708, a THS5641, a DAC908, and a TLC7524.

Embodiments of the subject matter and the functional operationsdescribed in this specification can be implemented in digital electroniccircuitry, or in computer software, firmware, or hardware, including thestructures disclosed in this specification and their structuralequivalents, or in combinations of one or more of them. Embodiments ofthe subject matter described in this specification can be implemented asone or more computer program products, i.e., one or more modules ofcomputer program instructions encoded on a computer-readable medium forexecution by, or to control the operation of, data processing apparatus.

While this specification contains many specifics, these should not beconstrued as limitations on the scope of the invention or of what may beclaimed, but rather as descriptions of features specific to particularembodiments of the invention. Certain features that are described inthis specification in the context or separate embodiments can also beimplemented in combination in a single embodiment. Conversely, variousfeatures that are described in the context of a single embodiment canalso be implemented in multiple embodiments separately or in anysuitable subcombination. Moreover, although features may be describedabove as acting in certain combinations and even initially claimed assuch, one or more features from a claimed combination can in some casesbe excised from the combination, and the claimed combination may bedirected to a subcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the embodiments described above should not be understoodas requiring such separation in all embodiments, and it should beunderstood that the described program components and systems cangenerally be integrated together in a single software product orpackaged into multiple software products.

Thus, particular embodiments of the invention have been described. Otherembodiments are within the scope of the following claims. For example,the actions recited in the claims can be performed in a different orderand still achieve desirable results.

The invention claimed is:
 1. An apparatus for generating an amplitudemodulation signal, comprising: a modulator control circuit configured togenerate a modulation control signal including transitions from a highamplitude to a low amplitude back to a high amplitude, the modulatorcontrol circuit is configured to provide the transition from the highamplitude to the low amplitude at a first slope, the modulator controlcircuit is configured to provide the transition from the low amplitudeto the high amplitude at a second slope, wherein the modulator controlcircuit is adapted to selectively configure at least one of the firstand second slopes to comply with a spectral mask; and a low dropoutregulator configured to receive the modulation control signal from themodulator control circuit and provide the amplitude modulation signalaccording to the modulation control signal.
 2. The apparatus of claim 1,further comprising a switchmode power amplifier responsive to theamplitude modulation signal, the switchmode power amplifier configuredto provide an AC output signal proportional to the amplitude modulationsignal.
 3. The apparatus of claim 2, wherein the modulator controlcircuit is adapted to selectively configure at least one of the firstand second slopes to reduce the spectral density of the AC outputsignal.
 4. The apparatus of claim 3, wherein the spectral density isreduced to comply with the spectral mask.
 5. The apparatus of claim 1,wherein the modulation control signal is configured to remain at the lowamplitude for a dwell time, wherein the modulator control circuitcontrols the length of the dwell time.
 6. The apparatus of claim 1,wherein the modulator control circuit is configured to receive a firstslope input, a second slope input, and a data input, wherein themodulation control signal is a representation of the data input, whereinthe first slope is controlled by the first slope input, wherein thesecond slope is controlled by the second slope input.
 7. The apparatusof claim 1, wherein the modulator control circuit is configured toreceive a modulation depth input, wherein a difference between the highamplitude and the low amplitude is controlled by the modulator controlcircuit according to the modulation depth input.
 8. The apparatus ofclaim 2, wherein the AC output signal is connected to an antenna.
 9. Theapparatus of claim 8, wherein the antenna is configured to output aninterrogation signal to an RFID circuit that responds to theinterrogation signal.
 10. The apparatus of claim 9, wherein theinterrogation signal is proportional to the AC output signal.
 11. Theapparatus of claim 9, wherein the RFID circuit is attached to one of amedical sponge and a medical instrument.
 12. The apparatus of claim 9,wherein the antenna is configured for use in a medical environment,wherein the modulator control circuit is adapted to selectivelyconfigure at least one of the first and second slopes to reduceinterference with electronic medical devices.
 13. The apparatus of claim12, wherein the modulator control circuit is adapted to selectivelyconfigure at least one of the first and second slopes to reduceinterference with a heart pacemaker.
 14. The apparatus of claim 1,further comprising a DC to DC switch configured to receive a DC inputvoltage and to provide a DC output voltage, wherein the low dropoutregulator is powered by the DC output voltage.
 15. The apparatus ofclaim 14, further comprising a control circuit connected the DC to DCswitch and the low dropout regulator.
 16. The apparatus of claim 15,wherein the control circuit is configured to establish a nominaloperating power level for the power amplifier via the amplitudemodulation signal.
 17. The apparatus of claim 15, wherein the controlcircuit is configured to maintain a minimum voltage difference betweenthe DC output voltage and an output of the low dropout regulator. 18.The apparatus of claim 17, wherein the minimum voltage difference is setat a particular level.
 19. An apparatus for generating an amplitudemodulation signal for an antenna configured for use in a medicalenvironment, the apparatus comprising: a modulator control circuitconfigured to generate a modulation control signal including transitionsfrom a high amplitude to a low amplitude back to a high amplitude, themodulator control circuit configured to provide the transition from thehigh amplitude to the low amplitude at a first slope, the modulatorcontrol circuit configured to provide the transition from the lowamplitude to the high amplitude at a second slope, wherein the modulatorcontrol circuit is adapted to selectively configure at least one of thefirst and second slopes to reduce interference with electronic devicesin the medical environment; and a low dropout regulator configured toreceive the modulation control signal from the modulator control circuitand provide the amplitude modulation signal according to the modulationcontrol signal.